If you’re working on a mobile app, save yourself hours of frustration and start with these mobile layout templates for iPhone, Android, HTC Touch and WebOS. Obviously, they’ll help you stay in that OS’s style and create a more polished looking app.
In my experience, they also let you figure out when certain things (that made a heck of a lot of sense in your hand-sketches) won’t work at all.
[BONUS: Here's a vector-based iPhone toolkit for Adobe Fireworks from Metaspark. It's great if you're a Fireworks junkie.]